Active research works have been made recently on optical materials. In the field of lens materials, in particular, the development of materials with good refraction properties, transparency, ready moldability, lightweight properties, chemical resistance and solvent resistance has been desired strongly.
Compared with inorganic materials such as glass, plastic lens is of such a lightweight that it hardly cracks, so such lens can be processed in various shapes. Therefore, plastic lens is rapidly spreading into not only eyeglass lens but also optical materials such as lenses for portable camera and pick-up lenses.
Following the spread, it is needed now to prepare materials per se with large refractive indices for the purpose of preparing thin lenses and preparing imaging devices in small shapes. For example, the technique for introducing sulfur atom into polymers (see for example JP-A-2002-131502 and JP-A-10-298287) and the technique for introducing halogen atoms and aromatic rings into polymers (see for example JP-A-2004-244444) have been researched actively. However, not any plastic material with a large refractive index and good transparency, which can be a glass alternative, has been developed yet. For optical fibers and optical waveguides, additionally, materials with different refractive indices are used in combination or a material with a distribution of the refractive index is used. So as to provide a material with variable refractive indices in various regions therein, the development of a technique for appropriately adjusting the refractive index has also been desired.
Since it is hard to raise the refractive index using organic matters alone, a report is issued about the method for allowing a resin to have a larger refractive index by dispersing an inorganic matter with a large refractive index in the resin matrix (see for example JP-A-2003-73559). So as to reduce the reduction of transmitted light via Rayleigh scattering, preferably, inorganic fine particles of a particle size of 15 nm or less are dispersed uniformly in the resin matrix. Because such primary fine particles of a particle size of 15 nm or less aggregate very readily, however, it is very hard to uniformly disperse such particles in the resin matrix. Taking account of the reduction of transmitted light in the optical path corresponding to the lens thickness, the amount of added inorganic fine particles should inevitably be limited. Therefore, it has never been achieved to disperse fine particles at a high concentration in a resin matrix without any reduction of the transparency of the resin.
Reports have also been issued, telling about a molded article mainly comprising a thermoplastic resin composition where ultra-fine particles of a number average particle size of 0.5 to 50 nm are dispersed, which is a molded article of a resin composition where the mean of the birefrigence of per 1 mm·optical wavelength is 10 nm or less (see for example JP-A-2003-147090), a thermoplastic material composition comprising an inorganic fine particle with a specific mean particle diameter and a specific refractive index, and optical parts using the same (see for example JP-A-2003-73563 and JP-A-2003-73564). These are also prepared by dispersing inorganic fine particles in resins. In terms of allowing fine particles to be dispersed at high concentrations in resin matrices without any reduction of the resin transparency, not any of the resulting products can exert sufficient performances.
Alternatively, for example, a method for melting and kneading together inorganic fine particles with the surface modified organically and acidic group-containing resins is reported as an organic-inorganic hybrid composition. The amount of added inorganic fine particles is at about 1% by mass, which is not sufficient (see for example JP-A-2004-217714). Additionally, an organic-inorganic hybrid composition prepared by binding together the surface-modifying group of inorganic fine particles and a resin via a linker is also reported (see for example JP-T-2004-352975). However, the procedures are laborious, such as the requirement of high temperature for forming the binding, while gelation is also concerned. Therefore, the resulting composition cannot exert a sufficient performance from the standpoint of moldability. Additionally, not any of these patent references includes descriptions about a transparent molded article usable as a lens with a large refractive index.